ARM: dts: am57xx-beagle-x15: Add GPIO controlled fan node

TPS gpio now controls a 5v 500mA TL5209 regulator which may be supply
a fan (such as AFB02505HHB) over J1 connector for various purposes.
Provide device tree node to enable the same.

Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>